Jason Short

I have never seen anything like this before....

I am helping a friend build a 396 for his car and am frustrated with a valvetrain geometry problem. I have measure pushrod length twice now, both on the intake and exhaust, full mock up. The problem is that the exhaust pushrod length is measuring .150" longer than the intake That suprises me since this is a standard LT1 block, stock ported LT1 heads, 977 springs/retainers (which I have used on many builds), and crane 11552 lifters. This is the first time I have use the lifters, but the engine they came out of ran the same length pushrods on both the I and E side.

Any thoughts? The only thing I can think of is that the cam has 2 different (accidently) lobe (not core) base circle diameters. ??

Has anyone run into this before? This is the first time I have ever seen anything like this (~15 engine builds).

Thanks, Jason

PatrickCarter

J, I assume stock valves too? If not are you sure you didnt get +.100 valves or something to that effect? The only other thing I can think of is what you said abotu the different lobe on the I and E side of the cam. Good luck

P

SStrokerAce

Yeah the valve length on the intake and exhaust valves is different. Most cams will keep the same base circle to get the same PR length.

You can order two different PR lengths... if it's a Comp PN just change the -16 to a -8

Bret

Jason Short

Yeah, stock valves.....which I didnt realize that they were different lengths. ?? The reason I am puzzled is that aside from this exact cam and lifters, all the other parts I have used in numerous builds....all stock or standard stuff. I am going to measure the lobe base circles to verify they are the same.

.150" difference seems huge to me

Thanks for the help guys

SStrokerAce

stock GM valves?

then the VJ was sunk into the seat badly on the exhaust....

You have a spring compressor to yank out a set of valves and measure them?

It could be the cam but thats not very common.

Bret

marshall93z

I know on a stock cam, there is about .013" difference in the lobes.

1.232" vs 1.245" if my memory serves me correctly.

Jason Short

Bret, the reason I dont think that is the issue is because these heads just came off of Coats' 383 last year. We had measured the pushrod length on that build (stock length), they came out all the same length. That motor only had a few thousand miles on it when we had to take it out due to an improper engine balance/vibration problem.

Since then, I did more porting on his heads and he had a good shop in Buffalo check them out, and a new valvejob. Maybe the valvejob on the exhaust side is quite a bit "deeper" than the intake? I didnt even think about that.

I checked out the cam lobe base circles and the *seem* to be the same diameter.

Holding a level over the valvetips (springs/retainers installed), they all are pretty true. Definatly not the .150" difference there. That leads me to believe that the valvejob is at basically the same cut depth.

Seems that would only leave the lifters, but the lifters came out of a motor that had all the same pushrod length.

Maybe I somehow jacked the measurement by having the lifter not quite on the heel. But, I check the intake on when the exhaust valve just starts to open, and the exhaust just when the intake valve fully closes. Same way I have set lash on my last 4 solid roller motors. ??

Thanks for the help
Jason

PatrickCarter

Hyd cam jason? Is it possible a couple of the lifters arent pumped up? Try priming the motor maybe?

P

SStrokerAce

Pat,

If they were hyd that would be one thing....

http://store.summitracing.com/defaul...+400078+300401

Bret

Mindgame

Jason,
Are you SURE you don't have a cam with two different base circles? Seen a number of mechanical roller cams with different bc's.

Other than that... I'm lost. You definitely have me thinking about everything that could cause it to be off though.

I have seen people run different length pushrods on the same base circle sizes just to try and get lift to a certain point at a certain time in the cycle. Takes lots of testing to find that though.

I'll chalk this one up with the Nazca Lines.

-Mindgame

Damon

The same way you laid a straight edge along the valve tips you should do that on the lifters, too. You want to do this on TDC, compression stroke, of the cylinder you are checking the lifters on (where both lifters are on the heel of the cam). If one is obviously sitting lower than the other (especially by a 1/10th of an inch or more- which you can easily see by eye) then you've got a cam with different base circles intake vs. exhaust. If you check one and think this may be the case I would definitely check ALL of the different cylinders in the same manner to make sure they all exhibit this difference in base circle diameter. If you only find a difference on one or two cylinders I'd start wondering if you just have a cam that was ground wrong.

Also check the depth of the cup inside the lifter body relative to it's top edge. Not all lifters are exactly the same in this measurement. I can't imagine how you could end up with one type of lifter on all the intakes and a different one on all the exhausts, but you're obviously going to have to check every possibility if you intend to sleep well at night.

Take all the sticks and group them together in your hand. Even them all up on the bottom against a good flat surface- glass tabletop or similar. Make sure they're all the same length. It's not uncommon to have them be off by maybe a hundredth or so, but anything off by a tenth or more will be very obvious.

Last thing that can be throwing you off.... rocker arms. Same on intake and exhaust?? You're not mixing 1.5s and 1.6s, even if they're from the same manufacturer, are you? Different rockers can have the pushrod cup at different "depths" relative to the fulcrum point.

If all that's the same then you just botched the measurement.

Jason Short

Thanks for the help guys...

Patrick...its a solid roller motor so no issue with the check valve/not pumping up.

Mindgame, yeah I am thinking the cam is the issue. On the cam card, I dont see anything specific that says there are 2 different base circles. ?? I will look at that further. Did you get my PM last week?

Damon, thanks for the info. I did check the lifters to see if the pushrod base cup was any higher/lower than the top of the body. All that looks exact. The rockers are all 1.52 Comp pro mags....brand new.

I am going to look into the cam further to see what I can find out.

Thanks again!
Jason

PatrickCarter

Check your email J